Conventionally, crude feedstocks are subjected to refining treatment including desalting, atmospheric distillation, vacuum distillation, and other steps to separate out various fractions, which are typically then subjected to further treatment to yield useful products. It is common, for instance, to use fractions such as naphtha and gasoil as feeds to a steam cracker, where they are pyrolyzed to ethylene, propylene, butylenes, and other molecules which can be oligomerized (e.g., from light olefins to heavier olefins), polymerized (e.g., polyethylene, polypropylene), and/or functionalized (e.g., acids, alcohols, aldehydes and the like).
Certain components in the crude are objectionable because they can negatively effect one or more of the refining steps. The nature and concentration of these objectionable components can affect the value of the crude.
Naphthenic acid, a generic name used for organic acids present in crude oils, is highly objectionable in the refining of crude. The presence of such acids can cause various problems in the refining process, particularly with respect to corrosion of equipment such as towers and heat exchangers. Extensive treatments using anti-corrosion additives and/or expensive metallurgy changes are sometimes required.
Total Acid Number, or TAN, is considered a measure of the naphthenic acid content of a sample. High Total Acid Number (TAN) crudes are typically much less valuable than crudes that do not have high acid concentrations. TAN, as used herein, is determined by ASTM method D-664 and takes the units of mg KOH/gm oil; for economy of description the method of measurement and units will not be repeated.
The primary effects of High TAN content in crudes or crude fractions on steam cracking include increased corrosion rates in the furnace facilities, inlet piping, upper half of the convection section, and primary fractionator bottoms due to organic acids in the feed; increased corrosion rates and waste water limitations in the recovery section due to acidic byproducts (e.g., CO2, light organic acids) and specialty additives (e.g, phosphorus) added by refineries to inhibit TAN corrosion. Such additives have been linked to increased coking rates and radiant tube damage in steam cracking furnaces from refinery distilled steam cracker feeds.
In the past, high TAN crudes could be avoided because of the abundance of other types of crudes. However, as the supply of crude dwindles, it become imperative that the aforementioned problems be addressed. Accordingly, it would be beneficial if an efficient and cost-effective method of treating/processing high TAN crudes could be devised.
Current commercial processes typically run crude through a refinery pipestill to separate the crude into various fractions, such as a gasoil fraction. The pipestill step (i.e., distillation) does not destroy a significant amount of TAN. The gasoil fraction, among others, may be subsequently steam cracked to yield light olefins. A basic material, such as an amine, may be injected to neutralize the remaining acidity (generally light aliphatic acids such as acetic and formic acids) in the steam cracked products.
U.S. Pat. No. 1,953,353 teach treatment of a feedstock comprising organic acids like phenols and naphthenic acids, or a distillate thereof, to a temperature treatment of between 600° F. (about 315° C.) and 750° F. (about 400° C.) under atmospheric pressure, preferably wherein the oil is a distillate that has first been subjected to a treatment with liquid sulfur dioxide. This heat-soak treatment may have been satisfactory at a time when energy was relatively cheap, but separate temperature treatments are to be avoided if possible.
U.S. Pat. No. 5,820,750 is also directed to removal of petroleum acids from crudes by thermal treatment. The treatment comprises heating the feed to a temperature of at least 400° F. (about 205° C.), preferably at least 600° F. (about 315° C.) preferably up to 900° F. (about 480° C.) for a period of time sufficient to reduce substantially TAN of the feed while constantly sweeping away inhibitors indigenous or formed during the decomposition, e.g., water vapor. According to the patent, this process is said to be useful for crudes with TAN above about 2, in order to reduce TAN to levels less than about 1.5.
U.S. Pat. No. 5,891,325 relates to a process for reducing organic acids comprising thermally treating a petroleum feed containing in a thermal reaction zone comprising a plurality of stages in series to produce a volatile organic acids containing hydrocarbon fraction and non-volatile hydrocarbon fraction, treating the volatile hydrocarbon fraction to neutralize the organic acids, and then blending the thus-treated volatile hydrocarbon fraction with the non-volatile hydrocarbon fraction. The thermal treatment is at a temperature of 400° F. (about 205° C.) to 800° F. (about 425° C.). Treatment to neutralize the organic acids includes the use of suitable salts such as CaO, Ca(OH)2, CaCO3, and magnesium analogs.
U.S. Pat. No. 6,086,751 teaches to reduce the naphthenic acid content of crude by a process comprising flashing crude to remove substantially all the water therein, followed by a thermal treatment in a reactor at 650° F. (about 340° C.) to 800° F. (about 425° C.).
U.S. Patent Application Nos. 2004/0069682 and 2004/0069686 are directed to upgrading of heavy petroleum oils that are typically not suitable for pipelining without the use of diluents. It utilizes pyrolytic reactors operating under conditions that result in a rapid distillation with coke formation, and is said to reduce TAN. The process comprises introducing a particulate heat carrier to interact with the feedstock, separating the vapors of the product stream from the heat carrier, and regenerating the heat carrier using a calcium compound.
The present inventors have surprisingly discovered that high TAN crudes and high TAN refinery cuts may be directly introduced into a steam cracker pyrolysis unit without the necessity of a heat-soak treatment and/or without the necessity of chemical treatment, to obtain a product from the steam cracker that is significantly reduced in TAN.